1. Field of the Invention
The present invention relates generally to light emitting devices and more particularly to light emitting semiconductor structures coated with phosphor.
2. Description of Related Art
Light emitting diodes (LEDs) are p-n junction devices that convert an incoming flow of electric energy into an outgoing flow of electromagnetic radiation. LEDs emit electromagnetic radiation in ultraviolet, visible, or infrared regions of the electromagnetic spectrum. The light emitted by an LED is distributed across a spectrum that is approximately 20-40 nm wide and has a peak emission wavelength defined by design details such as the crystal composition. As a consequence of the peak emission wavelength, a single LED p-n junction cannot emit white light, which is composed of spectral contributions from almost the entire wavelength range of the visible spectrum.
FIG. 1 shows an example of a white light emitting device including an LED and phosphor. LEDs that emit blue light are used with phosphors (luminescent material) to produce light emitting devices which emit apparently white light. U.S. Pat. Nos. 5,813,753 and 5,998,925, for example, disclose light emitting devices in which a blue LED is disposed in a reflective cup and surrounded by material including phosphors. In the exemplary device of FIG. 1, a portion of the blue light emitted by LED chip 10 and the red and the green light emitted by the phosphors as a result of a partial absorption of the blue light can combine to produce white light.
Usually, white light generated by sources such as the device illustrated in FIG. 1 is not uniform in color. For example, the generated white light may be surrounded by colored rings. This nonuniformity is a consequence of the variations in the thickness of the phosphor-containing material surrounding LED chip 10. The variations in the thickness cause spatially nonuniform absorption of blue light and emission of red and green light. In particular, thick regions of phosphor containing material absorb more blue light and emit more red and green light than do thin regions of phosphor containing material. The light from thick regions thus tends to appear yellow or display reddish and greenish blotches, and the light from thin regions tends to appear bluish. As illustrated in FIG. 1, light emitted in path b travels much further through the phosphor than light emitted in path a. When light strikes a phosphor particle, the light is either absorbed and re-emitted at a different wavelength or scattered by the phosphor. Light that travels a longer distance through the phosphor-bearing layer is more likely to be absorbed and re-emitted. Conversely, light that travels a shorter distance through the phosphor-bearing layer is more likely to be scattered out of the device without being absorbed and re-emitted. As a result, more blue light is emitted from regions of the device corresponding to short path lengths through the phosphor, and more red and green light or amber light is emitted from regions of the device corresponding to long path lengths through the phosphor.
FIG. 2 shows an exemplary attempt to counter the problem of nonuniformity of white light. The particular attempt involves an arrangement of a mass of phosphor containing encapsulant within a package or a phosphor loaded optical element interposed in the light exit path of the blue light LED within an extended package. For example, U.S. Pat. No. 5,959,316 to Lowery entitled xe2x80x9cMultiple Encapsulation of Phosphor-LED Devices,xe2x80x9d which is incorporated herein by reference, proposes depositing a transparent spacer over and around the LED prior to deposition of a uniform thickness of phosphor containing material. However, surface tension makes the shape and thickness of the phosphor containing material, often deposited as a liquid or paste (solids dispersed in a liquid), difficult to control. In addition, phosphor layer 6 must be separated from LED chip 10. As a result, the effective size of the light emitting device, i.e., the combined size of the LED chip and the phosphor layer, is much larger than the size of the LED chip alone. Since the optics used to control the light emitted from the source can grow geometrically with the source size, the large source size proposed by Lowery can present implementation difficulties. A method of producing uniform white light from LEDs without the implementation difficulties of the previous methods is needed.
The present invention provides a method of conformally coating a light emitting semiconductor structure, such as an LED chip, with a phosphor layer. The method involves electrically coupling a light emitting semiconductor structure to a submount, applying a first bias voltage to the submount, and applying a second bias voltage to a solution of charged phosphor particles. The electric field created by the two bias voltages induces the phosphor particles to deposit on the conductive surfaces. For example, the submount and the light emitting semiconductor structure coupled to the submount may be immersed in a solution of phosphor particles. In some embodiments, the solution may also contain a binder material that helps phosphor particles securely adhere to the conductive surfaces and to each other, and/or a charging agent that helps charge the phosphor particles.
If the light emitting semiconductor structure includes a conductive substrate, deposition of the phosphor layer can be limited to the surfaces of the light emitting device by coating the submount surface with an insulating layer before bringing all surfaces in contact with charged phosphor particles. If the light emitting semiconductor structure includes a nonconductive substrate, a conductive layer may be created on the surfaces where phosphor deposition is desired before the insulating layer is selectively deposited. The conductive layer may be created after the light emitting semiconductor structure is coupled to the submount. Alternatively, creating the conductive layer on the surfaces of the light emitting semiconductor structure can be completed as a part of the light emitting semiconductor structure fabrication process, before coupling the light emitting semiconductor structure to the submount. After creating the conductive layer and selectively depositing the insulating layer, the surfaces of the submount and the light emitting semiconductor structure are exposed to the solution of phosphor particles.
The electrophoretic deposition creates a phosphor layer of uniform thickness on all conductive surfaces which are electrically biased and put in contact with the solution of phosphor particles. In one embodiment, the uniform thickness phosphor layer produces uniform white light.